Mobilization and partitioning of rare earth elements in the presence of humic acids and siderophores

Abstract

Developing rare earth elements (plus yttrium, REY) as a group of environmental tracer requires comprehensive understandings in their geochemical behaviors associated with natural organic matter. Recent work highlighted the promotions on REY mobilization and cerium oxidation by siderophores during silicate dissolution, but the mechanism remained ambiguous. Here, we performed batch fluid-rock interaction experiments to explore the functions of siderophore desferrioxamine B (DFOB) and humic acids (HA) towards REY mobility and partitioning during REY-bearing ferrihydrite dissolution. To acquire in-depth knowledge of organic controls on REY, we used multiple strategies, including elemental, multispectral, and electrochemical analyses, to investigate the organic regulation on REY geochemistry. This study sheds light on the function of ligand-specific selectivity and solid-fluid organic molecular fractionation, primarily dependent on hydrochemical settings (pH, organic compounds, ionic strength, and oxicity). Our results confirm the catalytic oxidation ability of ligand, which forms DFOB-Ce(IV) (K = 1042, electrochemistry), producing positive Ce anomalies in solutions by ligand-driven redox shifting. Both HA and DFOB showed high affinities to HREY, and facilitated LREY/HREY partitioning. The mobilization of REY and the development of Ce anomalies were limited by HA coatings that modified surface properties and disturbed the approach of DFOB. Excess siderophores attack inert HA coatings, facilitating REY liberation and Ce redox activities. The release of REY and catalytic oxidation of Ce can be inhibited at high ionic strength or under oxygen deficiency. Our study reveals that natural organic matter significantly influences the fate of REY in iron oxides, and crucial for the biogeochemical cycles of REY in nature.